Blender as an image editor software?
I'd say it depends on what you want.

For something like photo-manipulation (by that I mean color grading, adding effects like vignetting, etc.), the compositor can work very well (and non-destructively). After all, it's almost what the compositor was designed for. The only major difference is that the images were rendered with a camera, not Cycles or some other render engine.



However for creating images from scratch with 2D techniques like painting, blender is probably almost certainly not the best around. And using blender's image editor it won't be non-destructive (though for something like this I'm not sure if a non-destructive workflow makes a lot of sense?).

Painting is not my strong suit, but there have been some pretty amazing things done with blender's painting tools. Also note that blender's painting tools recently got a nice upgrade as part of the GSoC.

That said, if painting is what you are looking for, you might be better off with a program designed for painting, like Krita or MyPaint.